My 2001 yukon 5.3l had an issue with overheating and amongst several other things we noticed a constant air bubble problem in the coolant resevoir, needless to say we decided we needed to pull the heads. So we pulled the heads and had them worked, resurfaced and so forth. When we got the heads back we used new gaskets, all new bolts, new lifters, did the knock sensors while in the area, also put a new EGR, and all new exhaust bolts, and a new thermostat as well. After getting everything put back together and refilled with coolant, im still having the same issue as before the repair! Still trys to overheat and still has a steady flow of air bubbling out of the coolant resevior... I did a compression test on cylinders as well and all the compressions were fine. Any suggestions on what to do to fix my car? Ive already spent close to 1200 bucks and dont wanna keep throwing money...Thanks!